The implementation of damage identification systems in machines is defined as condition monitoring. During their operating lifetime, machines undergo wear and tear due to friction and aging. This wear and tear is primarily located in bearings, seals, etc., and if not detected can lead to catastrophic failures. One such example is bearings in aircraft. To check reliability of rotating components, bearing assemblies are disassembled and thoroughly inspected for degradation at periodic maintenance intervals. This task is expensive and time consuming.
In many industrial applications, the bearing temperatures rise beyond the maximum operating temperature of the lubricants, leading to evaporation. In such conditions, the bearing failure can be hastened. Accordingly, the possibility of unusually high temperatures operation increases the need for frequent disassembly and inspection, thereby increasing maintenance costs.
The maintenance requirements and costs have led to the development of various wired and wireless sensors for on-line condition monitoring of bearings by measuring temperature, vibration, strain, and the like. Monitoring these parameters provides useful information about the bearing. Using such information, a corrective action can be taken before failure of the system. Bearing mountable thermocouple temperature sensors are commercially available and are attached to an outer race (outer diameter structure) of the bearing for temperature monitoring.
More specifically, a typical bearing includes an inner race, which is coupled to a rotating element, an outer race, which is coupled to a stationary element such as a frame, and bearing balls disposed between the inner race and outer race. The inner race can rotate with respect to the outer race upon the bearing balls. The bearing balls are typically supported in the space between the inner race and the outer race by a cage. Such bearings are well known in the art.
As mentioned above, thermocouple temperature sensors have been coupled to the outer race for monitoring temperature conditions in the bearing. The sensors are coupled to the outer race because of the difficulties of connecting circuits and sensors to a rotating element. One drawback to this monitoring method, however, is that it has been shown that the outer race temperature is not reliable indicator of bearing temperature. This can be due, in part, to the fact that the outer race is typically coupled to large metal frame elements that act as a heat sink, quickly dissipating the heat generated within the bearing. As a result, the measured temperature is not always truly reflective of the internal elements of the bearing.
One solution to the issue of inaccuracy of the thermocouple method discussed above has been proposed in U.S. patent application Ser. No. 134/747,433. In that solution, a wireless sensor using an L-C oscillator may be mounted on the inner race or cage of the bearing which communicates inductively with an external sensor circuit. As such, the L-C oscillator may be mounted on the rotating elements such as the inner race or cage because no wires or other circuitry need be attached to the rotating elements. While this solution overcomes issues with the prior art, it is limited to short distances between the L-C oscillator and the external sensor circuit. In addition, metal barriers between the L-C oscillator and the external sensor circuit can disrupt the sensor signal.
Accordingly, there is a need for a wireless sensor for condition monitoring in a rotating bearing element that avoids the problems of the prior art.
In addition to ball bearings, mechanical face seals require similar condition monitoring. Mechanical face seals are assemblies that provide a sealing surface for rotating parts having internal lubrication. Mechanical face seals are typically made up of silicon-carbide and carbon. In case of lack of lubrication, or thin fluid film rupture between seal stator and rotor, the sliding surfaces may run dry resulting in increased friction and higher seal face heat generation. This may result in increased fluid leakage and premature seal failure. In such cases, excessive seal face wear may occur. As a result, the seal face may become wavy, tapered, or heavily grooved and in the case of liquid hydrocarbon type fluids, coke deposits and fluid evaporation due to excessive heat etc. may be observed. It has long been established that seal face temperature is a good indicator of operation. Thus, it is essential to monitor seal interface conditions.
In prior art designs, a thermocouple was attached to the seal stator for condition monitoring. This method is often undesirable as it is difficult to mount wired sensors in millimeter-size spaces that exist between mechanical seal and a rotating shaft. In addition, when the mechanical seal is replaced after its useful life, wired sensors are difficult to replace without disconnecting the signal processing circuitry due to the fact that thermocouples are typically glued to the seal stator.